This invention relates to a high strength, precipitation-hardening nickel base alloy with improved toughness, which exhibits satisfactory resistance to stress corrosion cracking and hydrogen cracking under a corrosive environment, particularly under a corrosive environment containing at least one of hydrogen sulfide, carbon dioxide and chloride ions.
Since metallic construction members for use in oil wells, chemical plants, geothermal power plants, and the like are required to possess a high degree of strength and corrosion resistance, most prior art construction members are strengthened by means of solid solution hardening+cold rolling hardening. However, construction members to which cold rolling cannot be applied because of having a complicated shape cannot be strengthened by means of such a conventional method.
One of the conventional methods of improving the strength of a nickel base alloy which may be applied to a construction member of a complicated shape is to incorporate Ti and Al (or Nb) as alloying elements so as to cause the precipitation, during heat treatment, of an intermetallic compound mainly composed of Ni.sub.3 (Ti, Al), i.e. .gamma.'-phase, or an intermetallic compound mainly composed of Ni.sub.3 Nb, i.e. .gamma."-phase.
A typical prior art precipitation-hardening alloy of this type is a nickel-base alloy such as Inconel Alloy-718 (tradename), Inconel Alloy X-750 (tradename), Incoloy Alloy-925 (tradename). However, because the conventional alloy of this type is of a low Cr-high Ti-system and .gamma.'-phase which contains mainly Ti other than Ni precipitates, the corrosion resistance is not satisfactory. For example, Inconel Alloy-718 is a precipitation-hardening Ni-base alloy utilizing the precipitation of .gamma.'-phase and .gamma."-phase with the addition of Nb, Ti and Al. However, since it contains a relatively large amount of Ti and .gamma.'-phase which contains mainly Ti and Al other than Ni precipitates, the corrosion resistance is degraded,
Not only a high level of strength and toughness, but also improved corrosion resistance, namely improved resistance to stress corrosion cracking and hydrogen cracking are required for a construction material which is used under a corrosive environment containing at least one of hydrogen sulfide, carbon dioxide and chloride ions, and usually containing all three, such as found in oil wells, chemical plants, geothermal power plants, etc. A construction material which is used as a construction member for such use is desirably subjected to cold working when the material is used in the form of a plate or pipe in order to increase the strength thereof. However, when the material is in the shape of a valve, joint, bent pipe, etc. to which cold rolling cannot be applied, it must be strengthened by means of precipitation hardening. However, according to the findings of the inventors of this invention, the conventional precipitation-hardening alloy, most of which is a .gamma.'-phase precipitation-hardening Ni-base alloy with the addition of large amounts of Ti and A1, exhibits degraded resistance to corrosion.
For example, a nickel base alloy which exhibits improved resistance to stress corrosion cracking disclosed in Japanese Patent Laid-Open No. 203741/1982 contains 2.5-5% of Nb, 1-2% of Ti and up to 1% of Al, and is hardened mainly by the precipitation of .gamma.'-phase of Ni.sub.3 (Ti, Al) and .gamma."-phase of Ni.sub.3 Nb through ageing. However, since the amount of Ti is rather large, it is easily over-aged, precipitating an over-aged phase of an intermetallic compound of .eta.-Ni.sub.3 Ti with the corrosion resistance, particularly the resistance to hydrogen cracking being degraded markedly. Therefore, it is necessary to strictly limit heat treatment conditions as well as ageing conditions in order to improve the corrosion resistance of the alloy of this type.
Japanese Patent Laid-Open No. 123948/1982 discloses an alloy of a similar type containing 0.7-3% of Ti. This alloy also contains a relatively large amount of Ti, resulting in a degradation in corrosion resistance. Since a lower limit of Ti is set, it may be said that the precipitation of .gamma.'-phase of Ni.sub.3 (Ti, Al) is intended in that alloy.
An article titled "High-alloy Materials for Offshore Applications" by T. F. Lemke et al., OTC (Offshore Technology Conference) 4451, May 1983, pp. 71-72 states that Inconel alloys and Incoloy alloys may be used for oil wells. Though these alloys are of the (.gamma.'+.gamma.") precipitation hardening type, they contain a relatively large amount of Ti, and some of them contain no Nb.